Experience received from a task as a reward is multiplied by your xp rate
1. Talk to Sigmund on 1st floor (For American players this would be the 2nd floor). Ask him if he has any quests for you. He will tell you of recent damage to the castle cellar. While the Duke insists it is from an earthquake Sigmund says he believes it is the work of “some kind of monsters”. He asks you to talk with other people around town to find out if they saw anything.
2. Talk to the Cook (one floor down) and then go back to Duke Horacio.
3. Go to basement (trap door just north of where the cook stands in the kitchen) and use any pickaxe on the wall, then pick up the Brooch from floor in the cave. Next, go back to Duke Horacio and show him the brooch.
(don't forget to bring a light source such as a lit candle, or an oil or bulls-eye lantern before going through the wall!)
4. You will be asked to show the brooch to Reldo in the Varrock Palace library.
5. After talking with Reldo search the bookcase on the western wall for a goblin symbol book and, once found, read it.
6. Head to Goblin Village (using a Falador Teleport can help here) and talk to the Generals. Select the “It doesn't really matter” option to get the information you need. (You will receive both goblin emotes)
7. Go back to Lumbridge, and talk to Duke Horacio. He will tell you to investigate the caves further. Go back into the tunnel (Basement). See the map below for the correct path.
8. Talk to Mistag, and do your Goblin Bow Emote in front of him. After talking right click Mistag and click the “follow Mistag” option. You will return to just outside the Lumbridge basement entrance. Exit and then go upstairs to talk with Duke Horacio again.
9. Pickpocket Sigmund for key which we will need.
10. Go to the room just to the south and open chest with your key to claim H.A.M. robes:
Note: It might be wise to save the H.A.M. outfit you obtained in this quest as some items may have a future use and can be very hard to come by through pickpocketting H.A.M. members! The key will vanish after using it on the chest once and it cannot be obtained again!
11. Go to the H.A.M. cave (behind the castle in some building ruins). Open the trapdoor and climb-down:
12. Close to the front of the cave you will see a crate, search it to receive silverware:
13. Return to Duke Horacio then go to the basement and enter the tunnel again. This time (rather than threading your way through the maze) you can take a shortcut by right-clicking and choosing to follow Kazgar who will appear near the spot where you found the brooch. Once there, talk to Mistag and a cut-scene will appear leading to the end of the quest and the reward.